In modern times, most people know this word indirectly from Helen Bannerman's "The Story of Little Black Sambo", not directly from slave references as several have incorrectly mentioned here. It is controversial because some people feel it was racially motivated.
This is the origin of the word as far as I know. Two opposing ramps originally used for BMXers, but now used by snow boarders, roller bladers, skateboards and others. See also quarter pipe.
I heard they're building a new half-pipe down at the park this summer. Let's try it out but I'm not down with wearing all of the armor they are going to require.